1. Field of the Invention
The present invention relates to a capacitive ultrasonic transducer including an electret, a method of manufacturing the ultrasonic transducer, an ultrasonic diagnostic apparatus, and an ultrasonic microscope.
2. Description of Related Art
Ultrasonic diagnostics, which is performed by irradiating ultrasonic waves to a subject to make a diagnosis on a condition of the subject based on an echo signal from the subject, has been in common use. One of the examples of ultrasonic diagnostic apparatuses used in this ultrasonic diagnostics is ultrasonic endoscopes used in medical field.
Ultrasonic diagnostic apparatuses are used not only in the medical field but also in industrial fields in order to make a diagnosis on a presence or absence of defections such as a flaw, a crack, and a hollow appeared on a subject (sample). These ultrasonic diagnostic apparatuses are known as nondestructive inspection apparatuses and nondestructive flaw detection apparatuses.
Also known is an analytic method using a so-called V(z) curve, in which ultrasonic waves are irradiated to an object (sample) to evaluate an acoustic property of the object, thereby quantifying an elastic property of the object or evaluating structure of a thin film. Ultrasonic microscopes are known as such apparatuses for analyzing property of the object from the V(z) curve.
These ultrasonic diagnostic apparatuses and ultrasonic microscopes are provided with an ultrasonic transducer for converting electric signals into ultrasonic waves to transmit the ultrasonic waves and for receiving the ultrasonic waves to convert the ultrasonic waves into electric signals.
Conventionally, a piezoelectric element such as a ceramic piezoelectric material PZT (lead zirconate titanate) has been mainly used as the ultrasonic transducer. However, in recent years, attention is focused on a capacitive ultrasonic transducer (Capacitive Micromachined Ultrasonic Transducer; hereinafter referred to as a c-MUT) manufactured using a micromachining technology as disclosed in Japanese Unexamined Patent Application Publication No. 2005-510264.
The c-MUT is configured by including a pair of plate electrodes (parallel plate electrodes) facing each other sandwiching an air gap portion therebetween, and transmits and receives ultrasonic waves by vibration of a film (membrane) including one of the pair of electrodes. Since the c-MUT converts ultrasonic signals into electric signals based on a change in capacitance between the pair of electrodes at the time of receiving ultrasonic waves, it is required to continuously supply a DC bias voltage, which is a relative high voltage, between the pair of electrodes during the reception of ultrasonic waves.
In order to solve the problem, Japanese Patent Application Laid-Open Publication No. 2-52599 discloses a c-MUT (capacitive ultrasonic transducer) which unnecessitates application of a DC bias voltage by providing an electret (electret insulating film) between a pair of electrodes. In addition, in Japanese Unexamined Patent Application Publication No. 2005-506783 discloses an ultrasonic transducer including an electreted film provided directly on a pair of electrodes, in other words, on a transmitting side of ultrasonic waves.